The present invention relates to forming a light transmissive, porous metal oxide optical layer having a low index of refraction upon a vitreous substrate. More particularly the invention relates to providing an antireflective coating or layer on a light transmissive substrate, such as a cathode ray tube glass faceplate, or a glass solar collector. When light passes through the interface between two mediums of differing index of refraction, such as the interface between a cathode ray tube faceplate and air on the viewing side, there is a reflection of some portion of the light. This reflected light is effectively not transmitted and is a transmission loss. For soda-lime glass faceplates, this transmission loss can be of the order of 4-8% which reduces the efficiency of the device.
If this reflection can be eliminated or minimized, a corresponding increase in the efficiency of light transmission can be effected for the device. For a cathode ray tube, this means more of the phosphor-generated light will be available to the viewer, i.e., a higher screen brightness.
It is known that coating a transmissive substrate with a light transmissive film is one way of eliminating reflectance.
For a coated surface, the minimum reflection is given by ##EQU1## n.sub.c =index of coating n.sub.1 =index of medium
n.sub.2 =index of substrate
From the above equation it follows that, in an air or vacuum medium (n.sub.1 .perspectiveto.1) in order for the reflectivity to be zero (R.sub.m =0), the relation between the indices of the substrate and coating must be: ##EQU2## and the thickness of the film, t, must also meet the quarter-wave optical thickness requirement ##EQU3## where .lambda..sub.m =wavelength of minimum reflectivity
This means that an antireflective film on glass having an index of refraction of 1.52, must have an index of around 1.23 from the above. This low index requirement makes it practically impossible to design a single layer inorganic antireflective film on glass, since the lowest index inorganic material MgF.sub.2 has an index of 1.38 which reduces the minimum reflection to about 1.2% and has been widely used in the industry.
However, the problem can be attacked in another direction. Since the index of refraction of a material is related to its density, which can be lowered by introducing porosity, the index can also be lowered. It is required, however, that the pore size must be substantially smaller than the wavelength of the light and the pore distribution must be homogeneous in order not to affect the light transmission and cause scattering.
The density and index of refraction in these materials are related by: ##EQU4## where n and d are the index and density of the non-porous or coating material, and n' and d' are index and density of the porous material. The above equation can be written in terms of porosity as ##EQU5## where P=percent porosity.
In order to make a film with an index of 1.23, for example if the dense material is glass with an index of 1.52, a porosity of approximately 59% is needed. FIG. 1 shows how the index of refraction of TiO.sub.2, Al.sub.2 O.sub.3 and SiO.sub.2 varies with porosity. Transparent, porous metal oxide has been produced from clear solutions derived from metal alkoxides. Preparation of colloidal and polymerized solutions which produced continuous transparent oxide films and bodies at low temperature have been presented by the inventor in "Alumina Sol Preparation From Alkoxides" from The American Ceramic Society Bulletin, Vol. 54, No. 3, pp. 289-290, and "Monolithic Glass Formation by Chemical Polymerization", Journal of Material Science Vol. 14 pp. 1843-1849 (1979).